This paper describes a method for manufacturing luminescent cellulose fibers. Good optical properties of cellulose fibers under UV-C illumination were achieved by incorporating ZrO₂ (0.5 mol% of Eu³⁺) stabilized by Y₂O₃ (7 mol%) into the fiber structure’s particles. Fibers were obtained from 8 wt% cellulose solution in N-methylmorpholine N-oxide (NMMO) with the addition of a luminescent modifier in the range between 0.5 and 10 wt%. The physico-chemical and mechanical parameters and the structure of these fibers were examined.

The use of functional nanomaterials and their combination with organic polymers leads to the formation of advanced composites and hybrid inorganic–organic systems having unique properties. The use of such materials as anti-counterfeiting agents (invisible under ambient conditions) is crucial for modern security systems and for further identification of labelled items and documents in various industrial applications. Here, the fabrication of functional paper by in situ incorporation of luminescent cellulose fibres into its structure, during the formation of paper pulp, is shown. In order to produce luminescent fibres, an in situ modification is made during the spinning process, using lanthanide-doped upconverting nanoparticles (SrF₂:Yb³⁺,Er³⁺). At first glance (in daylight), the functionalized paper cannot be distinguished from its unmodified form. However, when exposed to invisible near-infrared laser light (975 nm), bright green illumination appears. The modified paper preserves good mechanical durability, which is important for its potential for further processing in documents and other commercially available forms. Protecting documents and various materials with this kind of paper may open new horizons in materials engineering, forensics, and other industrial applications.

Cellulose fibres modified with the luminescent inorganic compound were obtained by using N-methylmorpholine-N-oxide as a direct solvent. The fibres were cut and introduced to the unrefined and refined paper pulp samples. Schopper-Riegler value of the pulps were measured and the laboratory paper samples were formed. Observations under UV radiation showed that the cellulose luminescent fibres were randomly and even scattered in the paper and were well visible. Measurements of the paper properties such as tensile index, Elmendorf tear resistance, Bendtsen air permeability, Bendtsen roughness, folding endurance, bursting strength and tensile energy absorption were carried out and the scanning electron microscopy (SEM) analysis was used for the assessment of the paper structure and the bonding properties of the luminescent fibres. SEM analysis showed that the cellulose man-made fibres create microfibril network and have excellent compatibility with the paper. The results showed that cellulose man-made fibres with luminescent properties express similar properties to natural cellulose fibres (e.g. external fibrillation and bonding properties) which are useful for the purpose of secure paper production. Cellulose man-made fibres introduced to the paper do not have any disadvantageous impact on paper properties.

A new coating method was developed, based on cellulose regenerated from an N-methylmorpholine N-oxide (NMMO) solution applied to paper. In particular, the purpose of this study was to compare the effectiveness of the “deep coating” method using cellulose regenerated from NMMO solution and the classic pulp refining process as methods to improve paper mechanical properties and overall quality. The “deep coating” name comes from the fact that the coating process is combined with subsequent heating of the coating before its final solidification. As a result of this operation, deep penetration of the cellulosic solution into the paper structure occurs. This increases the contact surface of the solution with the cellulosic fibers, which increases the strength properties of the paper structure. It was found that the cellulosic coating increased the apparent density of the paper, the tensile strength, the elongation, the resistance to bursting, and the double fold number. However, the coating also decreased the air permeability and the tear resistance of the paper samples. Despite some technological difficulties, this method could be an attractive solution in the case of a need for additional functionalization of a paper structure.